1. Field of the Invention
The present invention relates to a scanning optical apparatus for scanning a light outputted from a light source on a prescribed surface.
2. Description of the Related Art
In image forming apparatuses such as printers, copiers, and facsimiles, for the purpose of writing an electrostatic latent image on an image supporter (hereinafter referred to as “photoreceptor drum”), a scanning optical apparatus which scans a beam light for writing an electrostatic latent image on said photoreceptor drum is employed.
Such scanning optical apparatus uses a polarizer such as a polygon mirror for transforming the beam light into a scanning light. The beam light from a light source is converged on the surface of the polarizer, then converged again on the photoreceptor drum by a lens (so called, f. theta. lens). That is, the beam light is coupled in relation to the surface of the polarizer and the photoreceptor drum, thereby correcting an optical face tangle error of the polarizer.
In recent years, so-called a tandem-type image forming apparatus is employed as an image forming apparatus having a full-color printing function, which forms toner (developer) images of each kind (color) on each of a plurality of photoreceptor drums, and then, superimposes these images on an intermediate transfer unit, thereby forming a full-color image.
In such a tandem-type image forming apparatus and the like, a plurality of light sources corresponding to each of a plurality of photoreceptor drums are used for writing the electrostatic latent image simultaneously on the plurality of photoreceptor drums. The beam light emitted from each of the plurality of light sources are collectively reflected by the photoreceptor drums, and transformed into a scanning light. Here, separation of each of the beam lights after being reflected by the polygon mirror is required in order to guide each of the beam lights to the corresponding photoreceptor drums.
As shown in FIG. 3, separation of each of beam lights L is typically conducted by providing angular differences with the incident angles at which Beam lights L fall on with respect to a cross section K perpendicular to the direction of a rotational axis 1a of a polygon mirror 1 (sub scanning direction). Accordingly, when separation of Beam lights L is required, Beam lights L often fall on the mirror surface of Polygon mirror 1 (more precisely, Rotational axis 1a of Polygon mirror 1), in an inclined state, not being perpendicular, with respect to the mirror surface.
In addition, the scanning direction of Beam light L by Polygon mirror 1 (the depth direction in FIG. 3) is hereinafter referred to as “main scanning direction”. Also, the direction perpendicular to the main scanning direction is referred to as “sub scanning direction”. The sub scanning direction is a direction along Rotational axis 1a of Polygon mirror 1, that is, up-and-down direction in FIG. 3.
However, when Beam light L falls on Polygon mirror 1 in an inclined state, the following problem occurs.
FIG. 5 is a schematic diagram for explaining a problem which occurs when Beam light L falls on Polygon mirror 1 in an inclined state. Hereinafter, as referring to FIG. 5, a problem of when Beam light L falls on Polygon mirror 1 in an inclined state is described in details.
As shown in FIG. 5(a), the reflecting surface of Polygon mirror 1 moves in and out in the radial direction of Polygon mirror 1 along with its rotation about Rotational axis 1a. Accompanying with such an in-and-out movement of the reflecting surface, the incident position of Beam light L also moves in and out in the radial direction of Polygon mirror 1, and, for example, the in-and-out movement for the amount of ΔX is generated.
On the other hand, as shown in FIG. 5(b), when Beam light L falls on the reflecting surface of Polygon mirror 1 in an inclined state, the position in the direction of Rotational axis 1a (sub scanning direction) at which Beam light L is reflected by Polygon mirror 1 changes according to the in-and-out movement of the incident position of Beam light L, and, for example, a change for the amount of ΔX is generated as shown in FIG. 5(b).
Here, the incident position change of Beam light L in the direction of Rotational axis 1a with respect to Polygon mirror 1 entirely equals to the incident position change of Beam light L in the sub scanning direction with respect to the photoreceptor drum, and as illustrated in full line in FIG. 5(c), the scanning path of Beam light L on a photoreceptor drum 2 is curved. Surely, it is ideal when the scanning path of Beam light L is linear along the main scanning direction as illustrated in a dashed line in FIG. 5(c). Such a curvature of the scanning path is called “bow”.
In order to maintain the image quality, the bow needs to be eliminated by, for example, an optical system (such as lenses) arranged in between Polygon mirror 1 and Photoreceptor drum 2, for causing distortion of an image to be formed.
Here, in a patent literature 1 (Japanese Unexamined Patent Publication Number 2003-149573), a method for eliminating the above-mentioned “bow” is disclosed. That is, the method disclosed in Patent literature 1 is to arrange in between a polygon mirror and a photoreceptor drum, a scanning lens with its cross section shape in the scanning direction of a beam light or in the sub scanning direction perpendicular thereto formed in an anamorphic aspherical shape defined independently as a function of distance in the main scanning direction from a light axis of the beam light, as well as a correcting lens formed with the tilt of its cross section in the sub scanning direction formed to be changeable according to the position in the main scanning direction.
However, manufacturing a complex lens such as an anamorphic aspherical shape lens requires a complex die machining, and is generally very difficult. Additionally, in a multi-beam image forming apparatus using a plurality of Beam lights L as shown in FIG. 3, the correcting lens is generally distributed one by one for each of Beam lights L. Accordingly, when the bow is eliminated by means of the shape of the correcting lens as described in Patent literature 1, it is required to change the shape of each of a plurality of correcting lenses, and thus, a problem that the number of dies for manufacturing the correcting lens increases still remains.
Consequently, this invention has been invented considering the foregoing conditions, and the purpose of this invention is to provide a scanning optical apparatus, which does not require a complex shape lens and is capable of eliminating the bow of the beam light even when the shapes of a plurality of correcting lenses are uniformed.